Can cross-wiring in a Mitsubishi multi-split system cause evaporator coil failure? – GreenBuildingAdvisor

I’m seeking expert opinions on whether a cross-wiring installation error in a Mitsubishi multi-split system could lead to evaporator coil failure. Here are the key details:

1. System: Mitsubishi multi-split with three indoor units connected to one outdoor unit via a branch distribution box in the attic of a shop/studio located in the Pacific Northwest in climate zone 4C.

   Indoor units:

       Studio: MSZ-FS18NA

       Exercise room: MSZ-FS09NA

       Office: MSZ-FS12NA

 

   Outdoor unit: MXZ-SM36NAMHZ

   Branch box: PAC-MKA32BC

2. Installation: Completed in March 2023 as part of new construction.

3. Construction final inspection: May 2024

4. Problem: In September 2024, due to poor room cooling and a flashing error code 14, a technician was called and he determined the system was leaking refrigerant. The system was shut off until repairs could be made. In October 2024, a technician determined that the exercise room’s evaporator coil (MSZ-FS09NA) was leaking. The system only had 10 lbs of R410a instead of the required 10 lbs.

5. Evaporator repair: The evaporator coil was replaced in January 2025, and the refrigerant was refilled. The system was restarted after being shut off since September 2024 when the leak was diagnosed. The leak repair took about 4 months, which the installing contractor attributed to delays in obtaining the replacement coil from Mitsubishi.

6. Observed issues: In February 2025, I observed the following:

Office unit (MSZ-FS12NA) was outputting air at 141 deg F despite a 63 deg F setpoint, with the room at 76 deg F.

Exercise room unit (MSZ-FS09NA) was outputting air at 57 deg F despite a 66 deg F setpoint, with room at 57 deg F.

7. Wiring error discovered in February 2025: I emailed my observations above to the installing contractor. Their technicians came out and discovered that the exercise room and office indoor units had been cross-wired since the initial installation in 2023. The exercise room unit was controlling the office unit’s output and vice versa. The technicians corrected the S1/S2/S3 wiring connections and the system appears to be operating normally for the last week.

Questions:

1. Could the cross-wiring error between the MSZ-FS09NA (exercise room) and MSZ-FS12NA (office) units contribute to or cause the evaporator coil leak in the exercise room unit?

2. How might the combination of cross-wiring and operating with low refrigerant levels impact the system over time, particularly considering the different capacities of the indoor units involved?

3. What specific damage could result from potential freeze/thaw cycles during cooling mode and overheating during heating mode in this multi-split configuration?

4. In your professional opinion, is it reasonable to attribute the coil failure in the MSZ-FS09NA unit to the installation error, considering the system specifications and observed issues?

Any insights would be greatly appreciated!